Printing plates may be imaged on a plate-making machine and then transferred to a printing press. Once on the printing press, the images from the printing plates are transferred to paper or other suitable substrates. It is important that images printed using a printing press be properly aligned with the substrate on which they are printed.
One conventional technique of aligning the printing plate on a press cylinder of a printing press involves using a reference edge and an orthogonal edge reference point of the printing plate to align the printing plate on a punching apparatus to form registration features (e.g. registration openings) in the printing plate. The printing plate may then be aligned on a press cylinder of the printing press with registration pins that project through each of the registration features. Needless to say, the images formed on the printing plate by a plate-making machine must be properly registered with the formed registration features.
FIG. 1 is a schematic depiction of a conventional plate-making machine 10 (also known as plate-setter 10) having an imaging drum 12 on which a printing plate 14A may be mounted. Plate-setter 10 has an imaging head 16 which can impart an image onto printing plate 14A. In this case, imaging head 16 is axially movable relative to imaging drum 12 (i.e. along the directions parallel to the axis of imaging drum 12 indicated by double-headed arrow 24). Imaging head 16 typically includes a radiation source (not shown), such as a laser, which emits one or more beams of laser radiation capable of imparting an image onto printing plate 14A. A controller 20 controls imaging head 16 and its associated radiation source in accordance with print image data stored in a memory 22, so as to image printing plate 14A. The Trendsetter™ plate-setters available from Eastman Kodak Company represent examples of plate making machines having the basic configuration shown in FIG. 1.
FIG. 2A shows imaging drum 12 of plate-setter 10 in greater detail. Imaging drum 12 has a plurality of registration pins 18A, 18B, 18C which project from its cylindrical surface 13. In this case, imaging drum 12 comprises three registration pins 18A, 18B, 18C, which may be offset slightly from one another around the circumference of imaging drum 12 to enable the registration of different printing plates. Different printing plates can include printing plates having different sizes. As shown in FIG. 2A, a reference edge 15A of printing plate 14A is brought into engagement with two registration pins 18A, 18B to orient printing plate 14A with imaging drum 12. Typically, printing plate 14A is rectangular in shape and reference edge 15A may be one of the “long” edges of printing plate 14A (as depicted in FIG. 2A). In this case, the shorter, orthogonal edge 19A of printing plate 14A extends around the circumference of imaging drum 12. In some cases a “long” edge of a printing plate can extend around the circumference of imaging drum 12. An edge detector (not shown) detects the position of a third reference point 11 on orthogonal edge 19A of printing plate 14A. Third reference point 11 is located at a fixed circumferential distance 23 relative to at least one of registration pins 18A, 18B, and 18C and is used to determine a registration position of printing plate 14A. Printing plate 14A is clamped onto imaging drum 12 using any suitable clamping system (not shown). Typically, clamping systems clamp regions of printing plate 14A in vicinity to reference edge 15A and in vicinity to an opposing edge of printing plate 14A (not shown) that is substantially parallel to reference edge 15A.
With printing plate 14A clamped and registered, imaging drum 12 is rotated about its axis in either or both of the main-scan directions indicated by arrow 26, while imaging head 16 is moved axially relative to imaging drum 12 (i.e. in the sub-scan directions indicated by arrow 24) while scanning radiation beams onto mounted printing plate 14A. Controller 20 controls the relative movement of imaging head 16 and imaging drum 12 and controls the radiation source in imaging head 16 in accordance with print image data 27 to impart an print image 17 onto printing plate 14A. In this case, it is desired that an edge 17A of print image 17 be created substantially parallel to reference edge 15A. The region 25 of printing plate 14A that is adjacent to reference edge 15A and the region (not shown) that is adjacent to the opposing edge of printing plate 14A are covered in part by the clamping system and are not imaged.
After being imaged on plate-setter 10, printing plate 14A is punched in a punching apparatus 50 as shown in FIG. 2B. Printing plate 14A is registered on punch table 52 of punching apparatus 50 by bringing it into engagement with two registration surfaces 18A′, 18B′ on its reference edge 15A and registration surface 11′ on its orthogonal edge 19A. Punch table 52 comprises a third registration surface 11′ that is located a circumferential distance 23 from at least one of registration pins 18A′, and 18B′. With printing plate 14A registered to surfaces 18A′, 18B′, 11′, punching apparatus 50 creates a number of registration features (not shown) in printing plate 14A. The registration features created by punching apparatus 50 may have a wide variety of shapes, sizes suitable for engagement with press cylinder of a printing press.
Once printing plate 14A is punched, reference edge 15A and the opposing edge (i.e. parallel to reference edge 15A) of printing plate 14A may be bent (not shown). As shown in FIG. 2C, printing plate 14A is then mounted on a press cylinder 62 of a printing press. A clamping system (not shown) which is used to mount printing plate 14A to press cylinder 62, may comprise registration pins which project through the registration features punched in printing plate 14A to secure printing plate 14A to press cylinder 62 in correct alignment. The clamping system may also use the bent edges of printing plate 14A (if present) to secure printing plate 14A to press cylinder 62. When printing plate 14A is securely mounted to press cylinder 62, the clamping system overlaps non-imaged region 25 of printing plate 14A (i.e. adjacent to reference edge 15A) and the non-imaged region adjacent the opposing edge of printing plate 14A (i.e. the edge parallel to reference edge 15A). In this manner, the clamping system of printing press (not shown) does not impede print image 17 on printing plate 14A. Print image 17 is then transferred to a substrate (not shown) by applying ink to printing plate 14A and rolling press cylinder 62 to bring inked print image 17 into contact with the substrate.
There are several problems associated with this conventional registration process. The two registration pins 18A, 18B are mounted in predetermined fixed positions and do not necessarily match the position and orientation of reference surfaces 18A′ and 18B′ on punch table 52. This can lead to inaccuracies in the formation of the various registration features in proper alignment with the images formed on printing plate 14A. For example, factors such as wavy printing plate edges and plate edge burrs can cause registration problems when each of the imaging actions taken by a plate-setter 10 and the registration feature forming actions taken by punching apparatus 50 employ different sets of registration surfaces. There are also reliability challenges in consistently and accurately loading the plate into contact with the registration features. It is also difficult to define sets of pins that allow a wide range of plate formats to be imaged whilst not interfering with one another.
Image sensors such as CCD cameras have been proposed to improve these registration problems. For example, in commonly-assigned U.S. Pat. No. 7,456,379 (Neufeld et al.) an edge detection system is described, based on using a CCD camera to image the edges of a printing plate perpendicular to the sub-scan direction. Based on the information so obtained, the image data is then adjusted to compensate for any misalignment between the plate and the drum on which it is loaded. In commonly-assigned U.S. Patent Application Publication No. 2008/0236426 (Cummings et al.) printing plate imaging techniques are described in which the locations of at least two points on a reference edge of printing plate mounted on an imaging drum are determined. The locations of two or more points are used to determine a transformation that is applied to image data to yield transformed image data which is in turn used to image the printing plate. The locations of the points can be determined by use of backlighting to avoid errors encountered in illuminating from the top.
There is a need in the printing industry for methods and apparatus capable of consistently and automatically determining an outer mechanical edge of a printing plate that is to be imaged.
There is a need in the printing industry for methods and apparatus capable of consistently and automatically determining an outer mechanical edge of a printing plate that is to undergo imaging forming actions.
There is a need in the printing industry for methods and apparatus capable of enhanced determination of an outer mechanical edge of printing plate with an image sensor.